Ammonium nitrate neutralization

ABSTRACT

A method for neutralizing nitric acid with ammonia comprising feeding nitric acid into a first aqueous reaction zone, feeding ammonia into a second aqueous reaction zone which has an inlet at the lower portion of the first reaction zone and is in circulating communication therewith, circulating the nitric acid downward within the first reaction zone into the inlet of the second reaction zone so that it is substantially diluted before entering the second reaction zone, whereby a predominent proportion of the nitric acid is reacted with the ammonia in the second reaction zone so as to form an ammonium nitrate solution, and removing the ammonium nitrate containing solution from the second reaction zone and recirculating the ammonium nitrate containing solution from the second reaction zone to the first reaction zone so that the solution and gases therefrom are substantially completely introduced into the first reaction zone with sufficient force so as to create a scrubbing turbulence of the gas as gas bubbles in the first reaction zone whereby soluble components of the gas are dissolved into the solution of the first reaction zone, maintaining the velocity of liquid within the first reaction zone flowing downward toward the inlet of the second reaction zone less than the terminal velocity of rising gas bubbles so that substantially all of the gas bubbles are permitted to rise and disengage from the first reaction zone and thereafter be recovered, and recovering the ammonium nitrate product from the recirculated solution before additional nitrate acid flows into the recirculated solution.

This is a division, of application Ser. No. 723,929, filed Sept. 16,1976.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a commercially feasible process and apparatusfor neutralizing nitric acid with ammonia whereby the "smog" formationusually associated with such neutralization is effectively avoided. Moreparticularly, the present invention is concerned with an apparatusconfiguration which substantially reduces the amount of ammonia, nitricacid and/or ammonium nitrate evolved into the atmosphere during theproduction of ammonium nitrate from nitric acid and ammonia.

2. Description of the Prior Art

In applicants' prior application Ser. No. 152,063 filed June 11, 1971,now U.S. Pat. No. 3,758,277 and Ser. No. 260,392 filed May 18, 1972, nowU.S. Pat. No. 3,870,782, applicants' disclosed an ammonium nitrateneutralizer which would significantly reduce the degree of "smog"formation which previously occurred during ammonia -nitric acidneutralization procedures. Those earlier studies resulted in developmentof apparatus and methods which succeeded in reducing ammonium nitrateemissions from what were previously considered to be good emissioncontrol levels, i.e. three to five pounds per ton of recovered productammonium nitrate down to 0.78 pounds of ammonium nitrate and 0.77 poundsof ammonia per ton of product ammonium nitrate. These reductions inemission levels represents a substantial improvement over the emissionlevels previously being suffered by prior art processes.

Anticipated changes in Government standards with respect to ammoniaemissions, however, are expected to require even better results forcontrol of ammonia emissions. Applicants at first considered that theirprevious apparatus could meet such increased standards. It wasconsidered that, by varying the acidity within the first and secondreaction zones, the degree of ammonia emissions could be reduced.However, when the acid content was varied, it was found that either therate of ammonia emission was not reduced, or it was found that the rateof nitric acid or ammonium nitrate emission rose significantly.

In the prior apparatus, a deflector was situated above the outlet of thesecond reaction zone, which was situated within the core of theapparatus. The core of the apparatus defined a first reaction zone,which was situated outside the core of the reactor, and thus outside thesaid second reaction zone. The vapors and liquids fountaining from thesecond reaction zone hit the deflector and the liquids were deflecteddownwardly into the second reaction zone. The vapors would pass aroundthe deflector and would pass upwardly toward the gas outlet. In arrivingat the present invention, it was first considered to situate thedeflector such that both the liquids and the vapors fountaining from thesecond reaction zone were deflected entirely into the first reactionzone. it was theorized that any ammonia gases emitting from the secondreaction zone would be brought into contact with the acid conditions inthe first reaction zone and hence the scrubbing action in the firstreaction zone would scrub out any ammonia. It was found, however, thatwhen this was tried, the level of ammonia nitrate and nitric acidemissions rose drastically to levels comparable to previous prior artequipment.

A need, therefore, continued to exist for a method and apparatus offurther reducing the levels of particulate ammonium nitrate, ammoniaand/or nitric acid evolved during the production of ammonium nitrate.

SUMMAY OF THE INVENTION

Accordingly, it is one object of this invention to provide aneconomically feasible nitric acid ammonia neutralization method whichminimizes the extent of smog formation and ammonia emissions toacceptably lower levels.

Another object of this invention is to provide a thermal siphon pressurepump neutralizer for accomplishing said method, which is simple indesign, and which does not require auxiliary scrubbing equipment toreduce the smog formation and ammonia emissions to acceptably lowlevels.

Briefly, these and other objects of the present invention, ashereinafter will become more readily apparent, can be attained by athermal syphon-pressure pump neutralizer being adapted for neutralizingnitric acid with ammonia, comprising a reaction vessel suitable forcontaining an aqueous reaction medium and having a gas outlet in itsupper end and a product outlet spaced below said gas outlet, at least onelongated fluid impervious cylindrical member positioned substantiallyvertically within said vessel so as to define a second reaction zonewithin said member and a first reaction zone outside said member betweensaid member and said vessel, the lower end of said cylindrical memberbeing an inlet being situated in said vessel such that said inlet ofsaid cylindrical member is spaced above the bottom of the vessel,ammonia inlet means leading into the bottom of said reaction vesselbeing in close proximity to but spaced below said inlet of saidcylindrical member, nitric acid inlet means leading into said firstreaction zone being spaced a predetermined distance above said inlet ofsaid cylindrical member and below the level of said product outlet ofsaid vessel, said apparatus being operable for neutralizing nitric acidwith ammonia by filling said vessel with an aqueous medium at least tothe level of said product outlet and introducing nitric acid into saidfirst reaction zone through said nitric acid inlet means and ammoniainto the reaction vessel through said ammonia inlet means, and dilutingthe concentration of the nitric acid before entering said secondreaction zone so that the major portion of nitric acid being introducedthrough said nitric acid inlet means will be neutralized with theammonia being introduced through said ammonia inlet means in said diluteconcentration within said second reaction zone, and whereby circulationmixing and turbulence of the aqueous medium, nitric acid and ammoniabetween said first reaction zone and said second reaction zone isfacilitated by a thermal syphon and pressure effect caused by the heatof neutralization and the density differential between the productsolution and the reacted solution such that water vaporized by said heatof neutralization will be discharged through said outlet of said secondreaction vessel and ammonium nitrate product will be recovered throughsaid product outlet of said reaction vessel, wherein the improvementcomprises said reaction vessel further containing a free gas zonebetween said reaction zones and said gas outlet being situated such thatgases disengaging from said first reaction zone will enter said free gaszone, and then into said gas outlet, deflector means situated above saidsecond reaction zone and in communication with said first reaction zonesuch that solution and gases evolved from the outlet of said secondreaction zone are substantially completely reintroduced into said firstreaction zone before entering said free gas zone above said firstreaction zone with sufficient force so as to create a scrubbingturbulence within said first reaction zone, whereby turbulent contactbetween said liquid within said first reaction zone and said gases willscrub the gases and cause dissolution of soluble components of saidgases into said liquid, such that gases emanating from said firstreaction zone into said free gas zone are substantially completelyscrubbed of soluble components.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein:

FIG. 1 shows applicants' previous nitric acid-ammonia neutralizer;

FIG. 2 shows an embodiment of the nitric acid-ammonia neutralizer of thepresent invention;

FIG. 3 shows another embodiment of the nitric acid-ammonia neutralizerof the present invention; and

FIG. 4 shows still another embodiment of the nitric acid-ammonianeutralizer of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will first be made to FIG. 1, which shows one embodiment ofapplicants' prior ammonium nitrate reactor disclosed in theabove-mentioned U.S. Patents, to enable complete understanding of theimprovement of the present invention. In the apparatus 1, a nitric acidsolution enters vessel 2 through entry port 3 containing spargers 7 insuch a manner that it flows into and downward through that volume ofreaction solution defined as the first reaction zone 5. A secondreaction zone 11 is defined by the elongated, fluid impervious, central,cylindrical member 10 positioned concentrically within vessel 2. Member10 is provided with an inlet 9 and an outlet 12. A deflector means 21 ispositioned in the free gas zone above the outlet 12 of the secondreaction zone 11. Member 10 is positioned within the reactor vessel 2such that when the vessel contains an aqueous reaction medium, inlet 9of member 10 will be below the level of the aqueous medium of the firstreaction zone, and outlet 12 will be above the level of the aqueousmedium. The variable distance "X" between central member 10 and ammoniainlet 17, at whose base is attached ammonia sparger 15, controls thesize of inlet 9 and consequently regulates te velocity of the aqueousreaction medium withdrawn from the first reaction zone through inlet 9up into the upward flowing solution of the second reaction zone withinthe central member. Ammonia inlet 17 can also be provided with a screen19. The device is also provided with gas exit port 23 above free gaszone 22, containing screening or filtering means 25, and reactionsolution exit port 27.

In the previous neutralization process, concentrated nitric acidsolution is passed into the first reaction zone 5 where the nitric acidis diluted. The aqueous solution from the first reaction zone 5containing nitric acid is withdrawn by the force of the fluid currentsin motion in first reaction zone 5 into the second reaction zone throughthe entry port 9 at the base of the central member. The nitric acidcontaining solution contacts the upward flowing ammonia gas in thesecond reaction zone 11 where the majority of the reaction occursbetween nitric acid and ammonia. The reaction is exothermic and causesthe solution to boil. The force of the rising ammonia gas and the heatevolved by the reaction combine to cause the liquid, by a thermal syphonpressure pumping effect, to fountain above the second reaction zone 11and be deflected by deflector 21 such that the liquids are brought backinto the first reaction zone 5 while the gases are permitted to movearound the deflector 21 and into the gas exit port 23.

Although that prior device did succeed in markedly reducing the degreeof ammonia and ammonium nitrate emissions into the atmosphere far belowany other prior art device, it seemed desirable to reduce still furtherthe level of ammonia emissions. On the belief that the degree of ammoniaemissions was a result of the particular acid level in the reactor,experiments were conducted to study the effect of the acid level onammonia emission. Raising the acidity of the reaction medium reduced thelevel of ammonia emission, however, in practice, the result wassubstantial increases in nitric acid and ammonium nitrate emissions.

A study of the liquid and gas flow patterns in a plexiglass model of thedevice described in U.S. Pat. Nos. 3,758,277 and 3,870,782 has now shownthat the recirculation of liquid through the second reaction zone 11 wasvery high and that the upward flowing gas-liquid mixture is diverted atthe top of the second reaction zone 11 back into the first reaction zone5. Since the liquid level in the first reaction zone 5 is below thedeflector 21, the gas disengages from the liquid, but when the liquidjet strikes the surface of the first reaction zone 5, gas is entrained,producing large amounts of bubbles.

The possiblity of scrubbing the gases of their ammonia content was alsoconsidered and it was attempted to bring the gases and liquidsfountaining from the second reaction zone 11 entirely into the firstreaction zone 5. This initially resulted in emission losses that wereeven greater than attained in the older prior art devices. Furtherstudies to determine why the losses had been so high led to theunderstanding that the entrained bubbles that entered the first reactionzone 5 were being carried downwardly by the thermal pumping action andinto the second reaction zone 11. Since the bubbles are composed largelyof superheated steam, nitric acid is steam stripped from the solutionand may react with ammonia to form ammonium nitrate microparticulate inthe exhaust stack. Moreover, the bubbles of superheated steam in thesecond reaction zone 11 impair the mass transfer of ammonia into theliquid phase. The understanding that the difficulty of emission lossesseemed to be a result of gas entrainment in the first reaction zone 5seemed to mitigate against the concept of introducing substantially allof the gases and liquids fountaining from the second reaction zone 11into the first reaction zone 5. The evidence tended to support a theoryof improving the gas disengagement of the gases from the fountainingliquids.

Nevertheless, it has now been found that if the product fountaining fromthe second reaction zone 11 is substantially completely diverted intothe first reaction zone 5, then the degree of ammonia, nitric acid andammonium nitrate losses can be decreased to a surprisingly low extent,if certain other factors are carefully controlled. In particular, it hasnow been found that if substantially all of the gases and liquidsfountaining from the second reaction zone 11 are deflected into thefirst reaction zone 5 with sufficient force to create severe turbulencetherein, the turbulence will serve to intimately contact the gases withthe acidic liquid medium thereby scrubbing soluble components from thegases. The ammonia present in the gases will be contacted with the acidto form ammonium nitrate which will be dissolved in the bath.

A critical element to success is the provision that the downwardvelocity of the liquid medium in the first reaction zone 5 must beadjusted so that down flow of gas bubbles to the inlet 9 of the secondreaction zone 11 is prevented.

It is the discovery of the present invention that the presence ofammonia, nitric acid and ammonium nitrate contaminants in the expelledwaste gases from ammonium nitrate reactor can be substantially reducedif the reactor is designed, or the recirculating liquid flow rates areadjusted, such that the gas which is to be expelled into the free gaszone 22 above the liquid reaction zones is deflected back into the firstreaction zone 5 to allow further turbulent scrubbing of the gas bubblesby the liquid medium in said zone before exiting the reactor via thefree gas zone 22. It is essential that the rate at which the medium inthe first reaction zone 5 flows downward toward the entry port 9 of thesecond reaction zone 11 be less than the terminal rise velocity of thegas bubbles which are deflected into the first reaction zone 5. Theterminal velocity of a gas bubble is that velocity attained by a risinggas bubble in the liquid medium, starting from a rest position. In otherwords the downward flow of liquid in the first reaction zone 5 must notbe of sufficient force to counteract the natural tendency of thedeflected gas bubbles to rise to the upper regions of the first reactionzone 5. The down flow velocity of the liquid must not carry thesubstantial portion of the gas bubbles down through the first reactionzone 5 to the entry port 9 of the second reaction zone 11. Thus, byallowing the deflected gas bubbles to be turbulently scrubbed in thefirst reaction zone 5, any ammonia and/or nitric acid within the gasbubbles can be further scrubbed from the bubbles, and possibly someammonium nitrate particles within the bubbles. As a result of thefurther turbulent scrubbing of the deflected gas bubbles in the firstreaction zone 5, the gas which rises from the first zone 5 into the freegas zone 22 and which is expelled from the reactor contains very littleammonia, nitric acid or particulate ammonium nitrate and is thereby freeof the objectionable smog common to gas effluents from ammonium nitratemanufacturing processes.

As determined from various experiments in which air bubbles wereexpelled into a liquid solution and allowed to rise, the terminal risevelocity is typically in the order of 0.55 ft./sec.. This is a bestestimated average value from several experiments and does not representa fixed, non-varying value since the terminal rise velocity is afunction of several factors such as the components and size of the gasbubble and the viscosity and density of the liquid medium. However, theterminal rise velocity of the gas bubbles in any given system can beeasily determined by conventional methods.

Any means by which the velocity of the downward flowing liquid medium inthe first reaction zone 5 is controlled to a value of less than theterminal velocity of the rising gas bubbles deflected into the firstreaction zone 5 can be employed in the present invention. For instance,referring to FIG. 2, the velocity V₁ in the first reaction zone 5 can becontrolled to a value less than the terminal rise velocity of thedeflected gas bubbles by controlling the rate at which liquidrecirculates from the first zone 5 into the second zone 11. Thecirculation of liquid between the first and second reaction zones isfacilitated by a thermal pumping and pressure effect caused by theexothermic heat of neutralization and by the density differentialbetween the product solution and the reaction solution. The apparatus ofFIG. 2 further differs from the conventional apparatus in that deflector21 is positioned above the second reaction zone 11 such that liquid andgases fountained from said second zone are substantially completelydeflected back into the first reaction zone 5 without allowing the gasto first pass into free gas zone 22. In this case, the outlet of thecentral cylindrical member 10 can be at or even above the level ofliquid in the reactor.

Another satisfactory means for controlling the liquid velocity in such areactor can be accomplished by substantially enlarging the volume of thefirst reaction zone 5 relative to the second reaction zone 11, therebydecreasing the downward liquid velocity in the first reaction zone 5.This velocity can be decreased in the first reaction zone 5, if desiredto a rate which approaches zero. However, decreasing the velocity tothat extent introduces disadvantages in that it makes control of the pHof the first reaction zone more difficult. Moreover, the low velocityrequires the use of the relatively massive pieces of equipment toachieve the production levels which are normally obtained at highervelocities.

Another approach to achieve turbulent scrubbing of deflected gas bubblesin the first reaction zone 5 without carrying the bubbles down to theentry port 9 of the second zone 11 can be achieved by differentmodifications of the reactor, all of which, in effect, accelerate thedownward flow of recirculating liquid medium in the lower section 5" ofthe first zone 5 relative to the downward rate of flow of the liquid inthe upper section 5' of the first zone 5. Of course, in allmodifications, the downward flow rate of the liquid in the upper section5' of the first reaction zone 5 must be less than the terminal risevelocity of the rising gas bubbles in the liquid medium. One embodimentof accelerating liquid medium flow in the lower section of the firstreaction zone 5 is shown in FIG. 3 wherein the outer wall of vessel 2 isprovided with an outwardly expanded portion 26 in the upper section 5'of the first reaction zone 5 of greater cross-sectional area than thearea of the narrower portion 28 in the lower section 5" of the reactor.Consequently, the downward flow velocity V₁ of reaction solution in thefirst reaction zone 5 in the upper section 5' is less than the terminalrise velocity of gas bubbles in the section which prevents bubbles frombeing transported down into region 5" of the first reaction zone 5 andallows sufficient time for gas/liquid disengagement.

The liquid in the first reaction zone 5 continues downward into section5" of the device, where its flow velocity is increased, and it contactsthe concentrated nitric acid entering the device through spargers 7 ininlet line 3. The distance between inlet line 3 and entry port 9 is notcritical. The only requirement is that good mixing of the enteringnitric acid in the reaction medium occur before the acid solution flowsinto the second reaction zone 11. The flowing liquid then enters thesecond reaction zone 11, which is confined by a fluid impervious centralmember 10, through inlet 9. The size of inlet 9 is not critical andshould be of sufficient size to permit an unimpeded flow of liquid intothe second reaction zone 11. The liquid entering the second reactionzone 11 is vigorously mixed with the upwardly flowing ammonia gas whichenters the second reaction zone through ammonia spargers 15 of inlet 17.Most of the reaction between nitric acid and ammonia occurs in thesecond reaction zone 11. The heat of neutralization released by theexothermic neutralization process, with the associated steam produced,promotes the circulation of liquid between the first reaction zone 5 andthe second reaction zone 11. The force of the injected ammonia gas alsoaids in promoting the circulation of the liquid through the zones.Reaction solution fountains out of the central member though outlet 12where it is recirculated to the upper section 5' of the first reactionzone 5. The gas which is evolved from the second reaction zone 11 isdeflected back into the upper section 5' of the first reaction zone 5 bydeflector 21 where turbulent scrubbing of the deflected gas bubblesoccurs. The gas bubbles which disengage and rise upwardly and out of thefirst reaction zone 5 pass into free gas zone 22 and exit the reactorthrough outlet 23. Reaction solution containing product ammonium nitratecan be withdrawn through outlet 27.

The level of the liquid in the first reaction zone 5 should be ofsufficient depth to prevent gas bubbles from being carried down into thelower region 5" of the first reaction zone 5. The deflector device 21should be at, or below, the liquid level in the reactor to deflect thegas or vapor fountaining from the second zone 11 without allowing theevolved gas to pass directly into the free gas zone 22, therebyeffecting the turbulent scrubbing of the gass bubbles in the liquidmedium of the first reaction zone 5. The deflector can be of any shapeor size so long as it serves the desired purpose of deflecting theevolved gases back into the first reaction zone 5.

Another deflector means for achieving the desired turbulentreintroduction of the gases and liquids into the first reaction zone 5is accomplished by simply raising the liquid level in the reactor to aheight above the opening 12 of the second reaction zone 11 a sufficientdegree so that the liquid level aids to deflect the liquids and gasesfountaining from the second reaction zone, back in the first reactionzone.

Referring to FIG. 3, the distance between the liquid level in thereactor and the base of the upper section 5' in the first reaction zone5 need only be of sufficient depth to allow vapor/bubble disengagementwithout permitting bubbles to be carried down into the lower region 5"of the first reaction zone 5 having an accelerated velocity V₂ relativeto V₁. Thus, V₁ in section 5' must be less than the vertical risevelocity of the gas bubbles in the upper section 5' of the firstreaction zone 5. The accelerated velocity V₂ in lower section 5" isachieved by decreasing the cross-sectional area of the lower section 5"relative to the cross-sectional area of the upper section 5'. Bydecreasing the cross-sectional area of the lower section 5", and thusdecreasing the volume of the reactor in this section, the size of thereactor in this section can be decreased while maintaining goodproduction levels of product ammonium nitrate. Decreasing thecross-sectional area of lower section 5" obviates the necessity of usinglarger apparatus to achieve the same production levels of ammoniumnitrate. The cross-sectional area of the upper section 5' of the reactoris greater than the cross-sectional area of the lower section 5" and canbe of any convenient area such that the velocity V₁ of downward flowingliquid in section 5' is maintained less than the terminal rise velocityof gas bubbles in section 5' to allow disengagement of the rising gasbubbles from zone 5.

Although the outer wall of the reactor of FIG. 3 is shown as taperinginward over section 29 at the transistional section between the upper 5'and lower 5" sections of the first reaction zone 5, this area does nothave to be tapered. The reactor can be structured such that there is anabrupt change in the diameter of the outer wall between the two regionsof the first reaction zone without any gradual sloping of the reactorwall. However, such a configuration would have the structuraldisadvantage of being weaker than a reactor whose outer wall is slopedor inclined over a distance. Moreover, in a reactor wherein the inclineof the outer wall is flattened to a horizontal surface, a relativelyquiet, non-turbulent and non-mixing region can form at the base of theupper section 5' of the first reaction zone 5 which can not exist in areactor whose outer wall is inclined over the transitional region. Morepreferably, transitional section 29 of the reactor is provided with aslope of 30° to 90°, especially 45° to 60°.

Another embodiment of the apparatus of the invention is shown in FIG. 4,which is essentially the reactor of FIG. 2, except that instead oftapering the outer wall to a smaller lower section 5" of the firstreaction zone 5, the outer wall is maintained continuously vertical andeither one or both of the walls which define the lower section 5" of thefirst reaction zone 5 are provided with a member(s) 30 which channelsthe flow of liquid in section 5" thereby providing an accelerated liquidflow of velocity V₂. Suitable flow rates V₂ of the reaction medium inthis and other apparatus embodiments range from 1 to 2.5 ft./sec.,preferably 1.5 ft./sec.

In the operation of the apparatus of the present invention, aqueousnitric acid is sparged into the first reaction zone 5 at a point belowthe upper section 5' of the zone to provide rapid and thorough mixingand dilution of the acid in the liquid medium. The concentration of acidused can range from highly dilute to very concentrated. For bestefficiency, the acid concentration ranges from 55% to 68% which is therange of commercial acid preparations. Concentrations in this rangepermit the production of ammonium nitrate solutions in concentrationsranging from 77% to 84%. Lower or higher concentrations may, however, beused. The nitric acid entry line must be positioned a sufficientdistance from the entry port 9 to the second reaction zone 11 in orderto achieve sufficient mixing and dilution of the nitric acid to at leasta concentration below 0.30% in the lower section 5" of the firstreaction zone 5 as it enters the second reaction zone 11. Preferably,the acid is diluted from 0.05% to 0.30%.

In operation of the device of the invention, an ammonia containing gasis introduced into the reactor through entry port 17, and it is allowedto rise through the second reaction zone 11 where the ammonia reactswith the nitric acid in solution to form ammonium nitrate. Any suitableammonia containing gas can be used, such as ammonia itself, or ammoniacontaining gases such as a very common source known as urea off gaswhich consists of ammonia, carbon dioxide and steam. The maximum rate offlow of ammonia into the reactor is a function of the diameter of thecentral member defining the second reaction zone.

Because the reaction between ammonia and nitric acid is exothermic, thegas and liquid medium issuing from the second reaction zone 11 are hot.The gas is deflected by deflection means 21, as hot bubbles, into thefirst reaction zone 5. The previously violent and turbulent conditionsprovide for very efficient scrubbing of the gas bubbles containingsoluble components in the liquid. The soluble components which can existin the gas bubbles include nitric acid, ammonia and ammonium nitrate.However, it is very important to control the mixing, because, withoutcontrol, such undesirable situations as achieving decreased ammoniaemissions in the exhaust gas with concomitant increased ammoniun nitrateemissions can occur. In the turbulent conditions which prevail in theupper section 5' of the first reaction zone, the hot gas bubblesconsisting substantially of steam and CO₂ would be expected to stripfree nitric acid from the solution which would react with free ammoniain the bubbles to form ammonium nitrate, which, in turn, would beevolved as a particulate material from the reactor. Careful control ofthis situation can be achieved by controlling the free acid content inthe upper section 5' of the first reaction zone 5 to a value as low aspossible. It is important to provide a low level of acid concentrationwhich provides a driving force for removing ammonia in the gas bubblesby reaction to form ammonium nitrate. Normally, the gas bubbles are toohot to allow dissolution of ammonia in the gas bubbles into the liquidreaction medium. Therefore, the necessary driving force for escape ofammonia from the gas bubbles is provided by the presence of lowconcentrations of nitric acid in the reaction solution. If there is nosuch driving force, ammonia will be lost to the atmosphere. If too muchacid is present, the nitric acid will be vaporized from solution becauseof the stripping action of the superheated steam. Usually, the nitricacid concentrations in the uppermost portion of section 5' of the firstreaction zone 5 ranges from 0.02%-0.05%, preferably 0.035%-0.05%, withan optimum value of 0.035%, although it could be as high as 0.15%. Thereason why the value of 0.035% is the optimum figure can be ascertainedby reference to the following diagram which is a graph of pH versus acidor base content of an ammonium nitrate solution. ##STR1## The optimumconcentration point provides an adequate amount of nitric acid in theupper section 5' of the first zone 5 while providing good control of thepH solution. At concentration levels somewhat less than that of theoptimum value, control of the pH of the solution is much more difficultbecause of the close proximity to the inflection point of the acid-baseneutralization curve where wide variations in pH occur at slight changesin acid or base concentrations. It is possible to conduct the process bymaintaining the pH of the solution in the upper section 5' of the firstzone 5 on the basic side. While excellent nitric acid and ammoniumnitrate recoveries are achieved, this is accomplished with major NH₃losses. Thus, an ammonia concentration level as high as 0.02% could bemaintained to attain these results. There are instances where largeammonia losses can be tolerated to achieve very small nitric acid orammonium nitrate losses. In order to achieve overall good NH₃, nitricacid and ammonium nitrate recovery, the above concentration ranges fornitric acid concentrations should be maintained.

The free nitric acid concentration in the lower section 5" of the firstreaction zone 5 at the entry port 9 to the second reaction zone 11 mustalso be controlled if good ammonia, nitric acid and ammonium nitraterecoveries are to be achieved. Usually, the acid concentration rangesfrom 0.05% to 0.30%, preferably 0.18% to 0.23%, most preferably from0.2% to 0.23%.

The design modification of the present invention has resulted in asubstantial reduction in total emissions over the emissions obtainedfrom the prior art apparatus. Typically, ammonium nitrate emissions of0.5 pounds per ton of product, and ammonia emissions of about 0.1 poundper ton of product can be achieved with the present apparatus. Thisrepresents approximately a 36% reduction in ammonium nitrate emissionsand an 85% reduction in ammonia emissions over the best prior apparatus.

Having generally described this invention, a further understanding canbe obtained by reference to certain specific examples which are providedherein for purposes of illustration only and are not intended to belimiting unless otherwise specified.

The data in Table 1 were obtained from the conventional ammonium nitratereactor of U.S. Pat. No. 3,758,277 which has an outer shell of constantdiameter. The data in Table 2 below were obtained from the reactor ofthe present invention as embodied in FIG. 3 which has an expanded outershell disposed above a more narrow, lower outer shell. The narrow, lowerouter shell of the reactor of the present invention had the samediameter as the constant diameter outer shell apparatus of theconventional reactor. The reactor capacities in both cases were the sameat 175 tons per day of ammonium nitrate. The ammonia containing gasadmitted into each reactor was a urea-off gas having the samecomposition of 45-60 wt.%NH₃, 15-25 wt.%CO₂ and 20-30 wt.% H₂ O. Thenitric acid solution admitted into both reactors was of the sameconcentration which ranged from 57-58% by weight HNO₃. The flow rate(Gallons per minute) of nitric acid solution into the reactors isdependent upon the production rate of ammonium nitrate desired and canbe determined from the formula: ##EQU1## The reaction temperature inboth reactors ranged from 250-270° F. which is relevant because atatmospheric pressure, this temperature is the boiling point of theammonium nitrate being produced. These temperatures are indicative ofthe solution concentration.

In the reactor of the present invention, the downward liquid velocity V₁in the upper section of the reactor was 0.45-0.5 feet per second, whilethe downward liquid velocity V₂ in the lower section of the reactor was1.5 feet per second.

                  Table 1                                                         ______________________________________                                        Conventional Ammonium Nitrate Reactor                                                      Prod.                                                            Production Rate,                                                                           Free     Emission,                                                                              (#/ton)Product                                 Tons/Day (100%AN)                                                                          Acid, %  NH.sub.3 NH.sub.4 NO.sub.3                              ______________________________________                                        176          0.098    0.77     0.86                                           165          0.096    0.46     0.82                                           182          0.098    0.11     0.95                                           182          0.070    1.10     0.50                                           182          0.093    1.40     0.80                                           Average               0.77     0.78                                           ______________________________________                                    

                  Table 2                                                         ______________________________________                                        Production Rate                                                                           Product            (#/ton) React.                                 Tons/day 100%                                                                             Free     Emissions,                                                                              Product Temp.                                  NH.sub.4 NO.sub.3                                                                         Acid, %  NH.sub.3  NH.sub.4 NO.sub.3                                                                     ° F                             ______________________________________                                        119         0.032    0.1       0.6     259                                    211         0.036    nil       0.5     259                                    199         0.033    0.2       0.7     259                                    205         0.031    0.1       0.6     257                                    146         0.030    0.1       0.6     259                                    205         0.049    0.1       0.6     253                                    205         0.035    0.1       0.6     253                                                Average: 0.1       0.6                                            ______________________________________                                    

Having now fully described this invention, it will be apparent to one ofordinary skill in the art that many changes and modifications can bemade thereto without departing from the spirit or scope of the inventionas set forth herein.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:
 1. In a method for neutralizing nitric acid withammonia wherein nitric acid is fed into a first aqueous reaction zone,ammonia is fed into a second aqueous reaction zone, which has an inletat the lower portion of said first reaction zone and it is incirculating communication therewith;said nitric acid is circulateddownward within said first reaction zone into the inlet of said secondreaction zone and is substantially diluted before entering said secondreaction zone; wherein a predominant proportion of the nitric acid isreacted with said ammonia in said second reaction zone so as to form anammonium nitrate solution; and wherein said ammonium nitrate containingsolution is removed from said second reaction zone, recirculated to saidfirst reaction zone and ammonium nitrate product recovered from therecirculated solution before additional nitric acid flows into saidrecirculated solution, the improvement which comprises: recirculatingthe ammonium nitrate containing solution from said second reaction zoneto said first reaction zone so that the solution and gases therefrom aresubstantially completely introduced into said first reaction zone insufficient force so as to create a scrubbing turbulence of said gas asgas bubbles in said first reaction zone whereby soluble components ofsaid gas are dissolved into the solution of the first reaction zone, andwherein the velocity of the liquid within said first reaction zoneflowing downward toward the inlet of said second reaction zone is lessthan the terminal velocity of the rising gas bubbles so thatsubstantially all of the gas bubbles are permitted to rise and disengagefrom the first reaction zone and thereafter recovered.
 2. Theneutralization method of claim 1, whereby the circulation for saidprocess is facilitated by a thermal pumping and pressure effect causedby the exothermal heat of neutralization of the nitric acid with theammonia and by the density differential between the product solution andthe reacted solution.
 3. The neutralization method of claim 1, whereinthe concentration of the nitric acid entering the first reaction zone isbetween 55 and 68%, and the concentration of the nitric acid enteringthe second reaction zone is less than 0.30%.
 4. The neutralizationmethod of claim 3, wherein the concentration of nitric acid entering thesecond reaction zone is 0.05% to 0.30%.
 5. The neutralization method ofclaim 1, wherein the solution and gases evolved from said secondreaction zone are substantially completely deflected downwardly intosaid first reaction zone with sufficient force so as to create ascrubbing turbulence of said gas as gas bubbles therein, wherebyturbulent contact between liquid within said first reaction zone andsaid gas bubbles will cause scrubbing and dissolution of solublecomponents within said gas bubbles into said liquid.
 6. Theneutralization method of claim 5, wherein the level of liquid in saidfirst reaction zone is above the level of the outlet from said secondreaction zone such that solution and gases evolved from said secondreaction zone are substantially completely deflected and brought intoscrubbing and turbulent contact with the solution in said first reactionzone, whereby dissolution of soluble components of said gas bubbles intosaid liquid occurs.
 7. The neutralization method of claim 5, wherein theoutlet from said first reaction zone is beneath the level of the outletfrom said second reaction zone, and wherein a deflecting means isprovided intervening said gas outlet which means substantially preventssolution or gases from entering said gas outlet and which deflects saidsolution and gases downwardly into said first reaction zone so as tocause scrubbing and turbulent contact with the solution in said firstreaction zone, thereby dissolution of soluble components of said gasinto said liquid occurs.
 8. The neutralization method of claim 1,wherein the depth of the liquid in said first reaction zone issufficiently deep to permit bubbles to disengage from said liquid,without said gas bubbles being carried by the downward flowing liquid inthe zone to the inlet of said second reaction zone.
 9. Theneutralization method of claim 1, wherein the basicity or acidity of theliquid in the first reaction zone where said gas is in turbulent contactwith the solution of the first reaction zone ranges from 0.02% ammoniato 0.15% nitric acid.
 10. The neutralization method of claim 9, whereinthe nitric acid concentration of the liquid in the first reaction zonewhere said gas is in turbulent contact with the solution of the firstreaction zone ranges from 0.02% to 0.15%.
 11. The neutralization methodof claim 1, wherein the velocity of said downward flowing liquid in thelower section of said first reaction zone prior to entry into saidsecond reaction zone ranges from 1 to 2.5 ft./sec.